The timing of the article, just two days before Mr Romney is sworn in as senator for Utah, has prompted some to speculate he is positioning himself as a challenger to Mr Trump for the Republican presidential nomination.

But he went on to say: "With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable. And it is in this province where the incumbent's shortfall has been most glaring."

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Mr Romney said he would support the president in policies he thought were in the best interests of Utah or the US but speak out against actions "that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions".

"The appointment of senior persons of lesser experience, the abandonment of allies who fight beside us, and the president's thoughtless claim that America has long been a 'sucker' in world affairs all defined his presidency down," he wrote.

"Trump's words and actions have caused dismay around the world."

The world "needs American leadership", he argued, and "the alternative... offered by China and Russia is autocratic, corrupt and brutal".

A fight for the future of Republicanism?

By Anthony Zurcher, BBC News, Washington

The on-again, off-again feud between Mitt Romney and Donald Trump is back on, although the president - for the moment - is shying away from open warfare.

That's a bit of a surprise, given then-candidate Trump unloaded with both barrels in 2016, when Mr Romney called him a fraud and a phony who should be opposed at every turn by Republican primary voters.

Perhaps the president realises he needs all the support he can get in the Senate, where another outspoken Republican, Jeff Flake, recently turned disapproving rhetoric into actions that stymied administration priorities.

Mr Trump would much prefer the new senator from Utah to follow the lead of other Republicans, who have paired the occasional criticism with lockstep support for the White House's agenda.

That may not be Mr Romney's game, however. Whether he is positioning himself to pick up the pieces after a possible Trump tumble or, perhaps, simply serve as a check on what he sees as the president's more dangerous impulses, the timing of this op-ed suggests this is an opening shot.

The two men in many ways represent the past and the present of Republicanism. Now they could be about to fight for the party's future.

How have other Republicans reacted?

Ronna McDaniel, chairwoman of the Republic National Committee and a niece of Mr Romney, described her uncle's article as "disappointing and unproductive".

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Mr Trump's manager for the 2020 presidential campaign went so far as to accuse Mr Romney of jealousy.

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Former White House press secretary Ari Fleischer said he was also "disappointed" in Mr Romney.

What kind of relationship do the two men have?

A complicated one. During the 2016 campaign, Mr Romney said Mr Trump had neither "the temperament nor the judgement to be president" while Mr Trump called Mr Romney a "choke artist" and called his bid for the presidency in 2012 "the worst ever".

When the US Congress reconvenes on Thursday, Mr Trump faces a new challenge on the domestic front - in November's mid-terms the Republicans strengthened their hold of the Senate but lost the House of Representatives.

More on this story:

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